A liquid crystal display (LCD) utilizes the optical and electrical anisotropy of liquid crystal molecules to produce an image. The liquid crystal molecules have a particular passive orientation when no voltage is applied thereto. However, in a driven state, the liquid crystal molecules change their orientation according to the strength and direction of the driving electric field. A polarization state of incident light changes when the light transmits through the liquid crystal molecules, due to the optical anisotropy of the liquid crystal molecules. The extent of the change depends on the orientation of the liquid crystal molecules. Thus, by properly controlling the driving electric field, an orientation of the liquid crystal molecules is changed and a desired image can be produced.
The first type of LCD developed was the TN (twisted nematic) mode LCD. Even though TN mode LCDs have been put into use in many applications, they have an inherent drawback that cannot be eliminated; namely, a very narrow viewing angle. By adding compensation films on TN mode LCDs, this problem can be mitigated to some extent. However, the cost of the TN mode LCD is increased. Therefore, a totally different driving means called IPS (in-plane switching) was proposed as early as 1974. Then in 1993, Hitachi Corporation filed its first US patent application concerning IPS, in which an IPS mode LCD was disclosed. Then In 2000, an improved driving means called FFS (fringe field switching) was proposed. The FFS is similar to the IPS except its first common electrode.
Referring to FIG. 9, a typical FFS LCD 100 includes a first substrate 110, a second substrate 130 opposite and parallel to the first substrate 110, a liquid crystal layer 150 sandwiched between the first and second substrates 110.
The FFS LCD 100 further includes a common electrode 111 formed at an inner surface of the first substrate 110 facing the liquid crystal layer 150, an insulating layer 112 covering the common electrode 111, a plurality of parallel pixel electrodes 113 formed on the insulating layer 112, a first alignment layer 114 covering the pixel electrodes 113, and a first polarizer 115 formed at an outer surface of the first substrate 110 far from the liquid crystal layer 150.
The FFS LCD 100 further includes a color filter 132 and a second alignment layer 134 disposed between the second substrate 130 and the liquid crystal layer 150, in that order from top to bottom, a second polarizer 135 formed at an outer surface of the second substrate 130 far from the liquid crystal layer 150. At least one of the substrates 110, 130 is made from a transparent material, such as glass. Original rubbing directions of the alignment layers 114, 134 are parallel to each other, and are identical to a polarizing axis of the polarizer 115. The pixel electrodes 113 and the common electrode 111 are made of the transparent material selected from the group consisting of ITO (Indium-Tin Oxide) and IZO (Indium-Zinc Oxide).
When no voltage is applied to the common electrode and pixel electrodes 111, 113, the long axes of the liquid crystal molecules is in the rubbing direction of the alignment layers. Because the rubbing direction of the alignment layers 114, 134 is the same as the polarizing axis of the polarizer 115, light beams passing through the polarizer 115 can pass through the liquid crystal layer 150, and polarizing directions of the light beams do not change. Because the polarizing axes of the polarizers 115, 135 are perpendicular to each other, the light beams cannot pass through the polarizer 135, and are absorbed by the polarizer 135. Thus the FFS LCD 100 is in an “off” state, and cannot display images.
As shown in FIG. 10, when a voltage is applied to the common electrode and pixel electrodes 111, 113, an electric field 120 is generated between the common electrode and pixel electrodes 111, 113. A direction of the electric field 120 is parallel to the first substrate 110, and perpendicular to the pixel electrodes 113 and the common electrode 111. The long axes of the liquid crystal molecules twist to align in the direction of the electric field 120. When light beams pass through the liquid crystal layer 150, the polarization state of the light beams is converted to match the polarizing axis of the polarizer 135. Thus the light beams pass through the polarizer 135 to display images, and the FFS LCD 100 is in an “on” state.
However, because the common electrode 111 and the pixel electrode 113 are both disposed adjacent to the first substrate 110, and the liquid crystal layer 150 has a certain thickness, it is difficult for the electric field 120 between the common electrode and pixel electrodes 111, 113 to grasp those liquid crystal molecules that are distal from the first substrate 115. Thus such liquid crystal molecules cannot be readily or fully twisted to a predetermined angle in the electric field 120, such that a viewing angle, a degree of chroma, and a transmission ratio of the FFS LCD 100 are decreased.
Therefore, a new LCD that can overcome the above-described problems is desired.